1. Field of the Invention
This invention relates to a stress cell for a scanning probe microscope (SPM) and its use for studying stress-related problems in material science research. More particularly, it relates to a sample stage which can be attached magnetically or otherwise to a scanning probe microscope, a stressing device mounted on the stage which has a loading clamp with arms that engage two slots on the stage and a force-sensor mounted near the center on the bottom of the stage. A wedge is placed on the sample stage between two clamp-arms. The clamps grip a sample by its edges, pulling it against the wedge. A motor-driven micrometer is fastened to the clamp, and pulls down on the sample to bend it against the wedge, providing a stressed sample the surface of which may then be imaged by conventional scanning probe microscopy techniques including atomic force microscopy and scanning tunneling microscopy.
2. The Prior Art
Problems related to the failure of materials under stress are important in the semiconductor and aerospace industries. For example, stress-induced dislocations and micro cracking cause failure in semiconductor integrated circuits and high strength materials. Various techniques are used for studying these problems. At the high resolution end for in-situ studies, stress related problems like microcracking, lattice structural changes and dislocation propagation under stress are studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the prior art, stress is often induced by generating a thermal gradient across the sample and examining the structural changes in the sample using SEM or TEM. Light optical microscopy and position sensitive photodetectors have also been used in some cases. These methods often require a high vacuum system because oxidation of the sample at high temperature and corrosion assisted cracking are enhanced by the methods used to induce stress. Although these methods can be used to study thermal stresses, they are not suitable for studying pure mechanical stress (such as film-edge induced stress, strain and misfit dislocations in doped lattices and heteroepitaxy, and stress problems of embedded structural elements). This is because of the temperature dependence of the material properties. A micromanipulator has been used to stretch a sample to generate stress while the sample was examined with optical microscopy, SEM or measurements of its electrical properties.
Optical microscopy is convenient but lacks high resolution. SEM has high resolution (better than 50 nm), but it also has high setup and equipment cost and must be performed in a vacuum. TEM has even better resolution than SEM but it requires a complex sample preparation procedure, a very thin sample and much more expensive equipment. The scanning tunneling microscope (STM) and atomic force microscope (AFM) have proved to be very powerful tools for surface science, being capable of atomic resolution even without the sample in an ultrahigh vacuum. However, the SPM has not as yet been applied to the study of stress related problems due to a lack of stability in the devices available to apply stress to a sample.